This invention relates to internal combustion and more particularly to small internal combustion engines of the type used to power lawn mowers, snow blowers, generators and the like.
Several types of ignition systems are known for small internal combustion engines. One such type is the capacitive-discharge ignition system, wherein a charge capacitor is charged from a current source such as a charging winding, and is discharged in response to the gating on of a silicon controlled rectifier (SCR) in series with the charge capacitor and the primary winding of an output coil. The gating on of the SCR controls discharge of the capacitor through the output coil, which triggers ignition firing.
Since the gating of the SCR in effect controls the timing of ignition firing, control of SCR gating may be used to retard ignition timing when the engine runs at low speeds upon starting up to 1200 rpm, and to advance ignition timing when the engine runs at higher speeds. Unfortunately, typical prior art SCR gating control techniques are complicated and expensive in that they often require additional trigger coils or a number of semiconductor switches. Moreover, such gating control circuits often provide for advancing in a plurality of steps, and not for continuous advance over a wide range of engine speeds.